1. Field of the Invention
The present invention relates to a process for producing a membrane-electrode assembly for polymer electrolyte fuel cells.
2. Discussion of Background
A polymer electrolyte fuel cell is constituted by a plurality of membrane-electrode assemblies stacked one on another via an electrically conductive separator having gas channels formed therein, wherein each membrane-electrode assembly comprises a polymer electrolyte membrane and electrodes (a cathode (an air electrode) and an anode (a fuel electrode)) provided on both sides of the polymer electrolyte membrane. Each electrode is constituted by a catalyst layer being in contact with the polymer electrolyte membrane and a porous gas diffusion layer disposed on the exterior side of the catalyst layer. The gas diffusion layer performs a function to diffuse air or a fuel in the electrode and a function to discharge water formed in the electrode.
The following processes are known as processes for producing membrane-electrode assemblies.
(1) A process wherein on a gas diffusion layer, a coating solution containing an electrode catalyst and an ion exchange resin is applied to form a catalyst layer thereby to obtain an electrode, and two such electrodes are subjected to hot pressing for bonding in such a state that a polymer electrolyte membrane is sandwiched between the two electrodes (Patent Document 1).
(2) A process wherein on a substrate film, a coating solution containing an electrode catalyst and an ion exchange resin is applied, followed by drying to form a first catalyst layer; on the first catalyst layer, a coating solution containing an ion exchange resin is applied, followed by drying to form a polymer electrolyte membrane; on the polymer electrolyte membrane, a coating solution containing an electrode catalyst and an ion exchange resin is applied, followed by drying to form a second catalyst layer; from a laminate comprising the first catalyst layer, the polymer electrolyte membrane and the second catalyst layer, the substrate film is peeled off; and the laminate and two gas diffusion layers are subjected to hot pressing for bonding in such a state that the laminate is sandwiched between the two gas diffusion layers (Patent Document 2).
However, in the case of the process (1), the catalyst layer is formed by applying the coating solution having an electrode catalyst and an ion exchange resin dispersed in a solvent directly on the gas diffusion layer, whereby part of the ion exchange resin or the catalyst layer is likely to penetrate into the gas diffusion layer to clog part of voids in the gas diffusion layer. Especially, the ion exchange resin tends to penetrate into the inside of gas diffusion layer with the solvent from the catalyst layer. As a result, the gas diffusion properties of the gas diffusion layers tend to deteriorate, and there will be a problem such that in a high current density region, the output voltage of the polymer electrolyte fuel cell tends to be inadequate.
Further, with respect the electrode obtained by the process (1), a little ion exchange resin is present in the catalyst layer, whereby during hot pressing, bonding between the catalyst layer and the polymer electrolyte membrane is not sufficiently carried out. As a result, the following problems are caused:
(i) the resistance of the interface between the catalyst layer and the polymer electrolyte membrane increases
(ii) the polymer electrolyte membrane tends to undergo swelling by humidification and shrinking by drying, whereby when humidification and drying are repeatedly carried out, the interface between the catalyst layer and the polymer electrolyte membrane tends to be easily peeled off, namely, the durability against a humidity change tends to decrease.
In the case of the process (2), the polymer electrolyte membrane is formed by applying the coating solution directly on the first catalyst layer, whereby part of the ion exchange resin is likely to penetrate into the first catalyst layer to clog many voids in the first catalyst layer. As a result, the gas diffusion properties of the first catalyst layer tend to deteriorate, and there will be a problem such that in a high current density region, the output voltage of the polymer electrolyte fuel cell tends to be inadequate.
Patent Document 1: JP-A-04-162365
Patent Document 2: WO02/005371